The GSM standard (Global System for Mobile Communication) is a widely deployed wireless cellular telephone standard using digital speech transmission. It has been adapted to provide wireless communication for geostationary circular orbit satellite systems as well as for terrestrial cellular networks. Signals within the system are transmitted using Time Division Multiple Access (TDMA) frames and superframes. A superframe structure in the GSM "full rate" format in a terrestrial system consists of 4.times.26 TDMA frames as illustrated in FIG. 1. In each row, frames 1-12 and 14-25 each contain eight traffic timeslots. Since neighboring base stations in a GSM system are not required to be synchronized, a complete frame (frame 13 in FIG. 1) is left idle to guarantee that the mobile can capture and decode a broadcast control channel (BCCH) signal burst from a neighboring base station regardless of the frame time offset between the base stations.
Speech and information data (such as fax or computer data) are not the only information communicated during a phone call. In satellite and terminated systems, control signals are typically exchanged between a control station (base station or satellite) and a mobile phone during a telephone conversation. The control signals allow the mobile phone and the network to monitor and control the communication link. When a call is in progress, GSM provides control channels to communicate control information between a mobile unit and its associated network. Each traffic channel is associated with a slow associated control channel (SACCH), which typically carries about two messages per second in each direction with a transmission delay of approximately 0.5 seconds plus propagation delay. The SACCH channel typically communicates non-urgent information such as measurement data used for handoff decisions. In FIG. 1, every 26th frame in the superframe structure contains SACCH information. Each SACCH message is interleaved over 4 SACCH bursts in each superframe. Each SACCH frame comprises 8 timeslots allowing one unique SACCH channel or each mobile link.
A second control channel for communicating control messages to and from a mobile phone during a telephone conversation uses the traffic channel. This control channel is called the fast associated control channel (FACCH) and carries urgent messages such as a handoff command where the base station instructs the mobile phone to switch to another frequency or timeslot. The fast associated control channel can also be used to authenticate a subscriber.
Each mobile unit is assigned a logical channel consisting of a particular one out of the 8 timeslots in each TDMA frame used for traffic information. According to the GSM standard, speech frames are generated every 20 milliseconds by a speech coder. With a speech coding rate of 13 Kbps, this corresponds to 260 bits per 20 millisecond speech frame. The speech bits are coded up to 456 bits. These 456 bits are divided into 8 groups of 57 bits each. Then, the 57 bits of one speech frame are interleaved with 57 bits of another speech frame. To these 114 bits, a 26 bit sync word, two 1 bit FACCH (fast associated control channel) flags, two sets of 3 tail bits and 8.25 bits to accommodate up/down ramping and guard time are added to form a TDMA slot comprising 156.25 bits. Each 20 mS block of coded data representing a segment of the speech waveform is spread over eight consecutive TDMA frames in a process known as block diagonal interleaving. Each eight frame interleaved block is half overlapped and merged with 4 frames of each of the adjacent speech blocks in order to fill each timeslot with bits that have come half from one speech frame and half from another speech frame. Each TDMA frame is then transmitted on a different frequency using frequency hopping to obtain the benefit known as interference averaging or interferer diversity.
The format of each GSM burst is illustrated in FIG. 2. An 8.25-bit guard and up/down ramping time is left between each burst. The up/down ramping of one burst may overlap with that of the adjacent burst but may not overlap with its other bits. The up/down ramping on the uplink (mobile) transmission is usually 4.25 bit periods, leaving a 4 bit period margin for time alignment errors between different mobile bursts as received at the base station. The base station sends SACCH commands to advance or retard mobile unit transmission timing to accomplish this function. GSM base stations have a fixed transmission timing and hence can use the whole 8.25 bit periods for up/down ramping.
The 3 tail bits allow the impulse response to the channel and modulation filter to terminate within the burst, and ensure that the end bits are demodulated with the same error probability as bits in the middle of the burst.
The flag bits on either side of a sync word indicate whether the previous or current 20 millisecond speech frame contains speech or FACCH information. One complete 20 millisecond speech frame has 8 associated flag bits in total, allowing the receiver to reliably decide whether the frame contains speech or FACCH information.
The sync word is a known bit pattern used to determine the characteristics of the transmission channel and to enhance demodulation. The sync word of 26 bits allows determination of five coefficients of a symbol-spaced, 5-tap model of the composite channel impulse response comprising transmit and receive filtering and physical channel. This allows each burst to be demodulated with no additional information from previous bursts. Because the characteristics of a physical channel are dynamic, the sync word is ideally located in the middle of the burst so that channel changes are less likely to affect the sync word than the information bits.
During FACCH transmission in conventional systems, speech or information data frames are not transmitted by the base station. That is, speech frames are "stolen" and replaced by the FACCH information. If the speech interruption is on the order of 40 milliseconds (i.e., a relatively short FACCH message), a user is unlikely to notice the missing speech frame(s). However, for longer delays (i.e., relatively lengthy FACCH messages), the degradation due to speech replacement becomes noticeable. It would be desirable to transmit FACCH information, including relatively lengthy FACCH messages, without noticeable degradation.